Aiming for a total greenhouse gas concentration equivalent to 650 parts per million of carbon dioxide would reduce global GDP by about 0.2% [the IPCC report on mitigation will say], whereas...550ppm would cost about 0.6% of global GDP...-- from Climate Talks Enter Final Phase, Richard Black, BBC online.
... many climate scientists now argue that only agreeing to keep below about 450ppm can prevent major climatic consequences. The IPCC draft says keeping concentrations at this level could cost up to 3% of GDP.
"I can tell you that the probability for achieving 450ppm in anything approaching the world as it now is is almost impossible," commented Professor Stephen Schneider from Stanford University in California, who helped draft the IPCC's first report this year on the science of climate change. "But a temperature rise over 2-3C leads to potential mass extinctions, serious problems with coasts, mountain glaciers disappearing, melting ice sheets... and one has to talk about stabilisation at 450-550ppm range to have a better than 20-30% chance of preventing that."